The invention relates to the hot-pressing of powders to precise shape. Hot-pressing of ceramic materials has been accomplished for tens of years by axially loading a die, usually graphite, containing a suitably active powder which only densifies under the influence of heat and pressure. Because of the nature of the process, and the characteristics of the powder, relatively simple cross sections are possible. If one attempts more complex geometry, shapes can be formed; these, however, suffer from non-uniform density and anisotropic physical and mechanical properties.
Gas turbine development has progressed over the years to the point where metals and alloys have reached their limit, even when expensive cooling air is used to keep properties from degrading. It has become essential to look at ceramics for an answer. However, because of the above limitations, historically, hot-pressed-to-shape articles have tended to be dismissed as viable components for gas turbine application.
The present invention provides a process which allows for the hot-pressing of complex geometric shapes, in particular shapes required for automotive, aircraft, and stationary power, gas turbines. When a material is compacted, a normal molding ratio exists whereby, say a 2 inch thick preform can be compacted to 1 inch and its bulk density increased from 50% to 100% of theoretical. If materials acted hydrostatically and would flow into cavities, such as with the case of pressure casting of molten metals, any shape within reason could be made. Ceramic materials act like damp sand, however, bridging from one particle to the next, densifying mainly by solid state diffusion and migration of holes.